We propose to combine electrophysiological measurements and radioisotope flux measurements to study the control mechanisms of the resting potential of the nerve membrane. At present, very little is known about the transport mechanisms of the resting current (which determines the resting potential). Our preliminary studies have indicated that the resting potential is controlled by a new type of voltage-gated resting channel which differs in properties from the delayed rectifier K channel. The specific aims of this proposed research are: (1) To identify and to characterize the membrane pathways which control the membrane current at the resting state; (2) To thoroughly test the existing electro-diffusion theory (i.e. the Goldman-Hodgkin-Katz (GHK) equation) by simultaneously measuring membrane potential, membrane conductance and membrane permeabilities as a function of ionic concentrations. These experiments will be done under the condition that the excitable ionic channels and the ion pumps are blocked; (3) To develop an improved theoretical model of the resting potential by incorporating the observed transport properties of the resting pathways into the electro- diffusion theory. We hope this research will be able to provide a better model which will explain the experimental findings more satisfactorily than the GHK equation. The squid giant axon will be used as the primary biological model. This preparation has several technical advantages. For example, using an internal perfusion technique, both the internal and external ionic environment of the axon can be controlled at will. The unidirectional isotope-labelled ion fluxes can also be measured directly with little ambiguity. Furthermore, such measurements can be done at a constant membrane potential by using voltage-and space-clamp. Pharmacologic interventions are available to block excitable ion channels and ion pumps. With our specially designed apparatus, a number of experimental parameters can be measured simultaneously (e.g. membrane conductance and 42K efflux at a given membrane potential). Such simultaneous measurements will allow us to thoroughly test the theoretical model by comparing data obtained from different methods.